Circuit and method of operating a laser diode

ABSTRACT

An embodiment circuit includes a diode having a first terminal coupled to a first reference voltage; a first controllable switch coupled between a second terminal of the diode and a second reference voltage; and a capacitive element having a first terminal coupled to the first reference voltage and a second terminal controllably coupled to the second terminal of the diode.

TECHNICAL FIELD

The present disclosure relates generally to diodes, and, in particularembodiments, to a circuit and a method of operating a laser diode.

BACKGROUND

Laser diodes are widely used in high speed data communications.Modulated light from a laser diode may be used to carry information. Themodulated light may be transmitted through optical fibers or across freespace. For some data formats, when a laser diode emits light the datavalue is considered a logical one, and when the laser diode is off (e.g.where no light is emitted from the laser) the data value is considered alogical zero.

Vertical cavity surface emitting lasers (VCSELs) are one type of laserdiode used in data communication networks. Drive circuitry may provide aVCSEL with sufficient current to turn on, thereby causing the VCSEL toemit light. Likewise, the drive circuitry may remove or prevent currentfrom flowing to or through the VCSEL, thereby causing the VCSEL to turnoff and not emit light.

The ability to drive a laser diode to generate a light intensity outputhas many applications. For example, generating a light intensity outputenables the laser diode to encode information for transmission within anoptical output signal. Generating a light intensity output also enablesthe laser diode to encode timed events (e.g. where a positive edge of apulse denotes a start time) such as in time-of-flight detection where areceived signal is used to determine and resolve distances between atarget object and the laser diode. As signaling frequencies in highspeed data communications increase (e.g. in excess of 1 GHz), it may bedesirable to provide a method and a circuit for driving a laser diode tosupport robust optical communications even with such increases insignaling frequencies.

SUMMARY

In an embodiment, a circuit includes a diode having a first terminalcoupled to a first reference voltage; a first controllable switchcoupled between a second terminal of the diode and a second referencevoltage; and a capacitive element having a first terminal coupled to thefirst reference voltage and a second terminal controllably coupled tothe second terminal of the diode.

In an embodiment, a circuit includes a current source; a laser diodecoupled between a first terminal of the current source and a firstreference voltage; and a first switch controlled by a first controlsignal and coupled between a second terminal of the current source and asecond reference voltage, the second reference voltage being less thanthe first reference voltage. The circuit further includes a capacitiveelement having a first terminal coupled to the first reference voltage;and a second switch having a first terminal coupled to a cathode of thelaser diode and a second terminal coupled to a second terminal of thecapacitive element. The second switch may be configured to couple thesecond terminal of the capacitive element to the first reference voltagein response to a second control signal being at a first voltage, and tocouple the second terminal of the capacitive element to the cathode ofthe laser diode in response to the second control signal being at asecond voltage different from the first voltage.

In an embodiment, a method includes forward-biasing a laser diode inresponse to shifting an amplitude of a first control signal provided toa first switch from a first amplitude to a second amplitude anddecoupling a terminal of a capacitive element from a cathode of thelaser diode in response to holding an amplitude of a second controlsignal provided to a second switch at the first amplitude when shiftingthe amplitude of the first control signal to the second amplitude. Themethod further includes shifting the amplitude of the first controlsignal from the second amplitude to the first amplitude to preventcurrent from flowing through the forward-biased laser diode; andshifting the amplitude of the second control signal from the firstamplitude to a third amplitude to couple the terminal of the capacitiveelement to the cathode of the laser diode.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 schematically shows a circuit including a laser diode, a drivercircuit, and a snubber circuit, in accordance with an embodiment;

FIG. 2 shows an example control signals respectively provided to thedriver circuit and the snubber circuit shown in FIG. 1, in accordancewith an embodiment;

FIG. 3 shows a comparison of the output currents of the laser diodeshown in FIG. 1 with and without the snubber circuit, in accordance withan embodiment;

FIG. 4 shows an example of a level shifter that may be used to generatethe control signals shown in FIG. 2, in accordance with an embodiment;

FIG. 5 shows a simulation plot of the control signals generated usingthe level shifter shown in FIG. 4, in accordance with an embodiment;

FIG. 6 shows a flow diagram illustrating a method of operating a laserdiode, in accordance with an embodiment.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of various embodiments are discussed in detailbelow. It should be appreciated, however, that the various embodimentsdescribed herein are applicable in a wide variety of specific contexts.The specific embodiments discussed are merely illustrative of specificways to make and use various embodiments, and should not be construed ina limited scope.

Laser diodes are widely used in high speed data communications. Laserdiodes may also be used in a time-of-flight ranging sensor forautofocus, proximity sensing, and object detection in robotics, dronetechnology, and internet-of-things (IoT) applications. Laser diodes maybe driven by driver circuits, and, consequently, operation of suchdriver circuits may be vital to the performance of a photonic systemthat includes such laser diodes. As an example, driver circuits may needto effectively and robustly drive laser diodes such that optical outputcurrents from such laser diodes exhibit steep rising edges, steepfalling edges, and minimal ringing. Such optical output currents allowlaser diodes to support high signaling frequencies (e.g. greater than 1GHz) and also allow these laser diodes to be used to encode timedevents, such as time-of-flight detection and distance resolution (e.g.in proximity sensing or objection detection applications).

Laser diodes may generate light (e.g. infrared light) using an activelaser medium that is formed by a p-n junction. The p-n junction mayaccumulate charged carriers during operation of the laser diode, andsuch charge accumulation can detrimentally affect the rise time, thefall time, and damping of optical output currents generated by the laserdiode. For example, the optical output current generated by the laserdiode may have slewed edges and may exhibit ringing as a result of RLCresonance of parasitic elements. These characteristics of the opticaloutput current can result in a current pulse shape that, in turn, canreduce the efficacy of the laser diode in discriminating multiple targetdetermination and can make tight windowing in optical communicationsmore difficult (e.g. in a time window where ambient noise isdetermined).

As such, it may be desirable to provide a method and circuit for drivinglaser diodes in a way that compensates for the accumulation of chargedcarriers in the laser diodes and that decreases the rise time, the falltime, and ringing of optical output currents generated by the laserdiode.

An embodiment circuit provides these desirable characteristics. Inparticular, an embodiment circuit draws out remaining charged carrierswithin a p-n junction of a laser diode, leading to a rapid turn off timefor the laser diode. The embodiment circuit also serves to damp RLCparasitic ringing of an optical output current generated by the laserdiode at turn off. The embodiment circuit further allows for a non-zerovoltage drop across the laser diode, below the diode's forward biasingvoltage, with the laser diode held in a state where it is effectivelynot emitting light, thereby reducing the need to pre-charge anyparasitic node capacitance that exists within the laser diode and itsdriving circuits (that being any unnecessary excess voltage delta,Q=CV), thus leading to a faster turn-on response for the laser diode.The embodiment circuit additionally allows for charge recovery, wherethe damped charge removed from the p-n junction of the laser diode ispumped back to the power supply.

FIG. 1 schematically shows a circuit 100 including a laser diode 102, adriver circuit 104, and a snubber circuit 106, in accordance with anembodiment. In some embodiments, the laser diode 102 may be asemiconductor laser (e.g. an electrically pumped semiconductor layer) inwhich an active laser medium is formed by a p-n junction of asemiconductor diode. In some examples, the laser diode 102 may be avertical-cavity surface-emitting laser (VCSEL). Other examples of thelaser diode 102 include quantum well lasers, quantum cascade lasers,interband cascade lasers, and vertical external-cavity surface-emittinglasers (VECSELs), to name a few.

Laser diode 102 has a first terminal 102 a (e.g. the anode) and a secondterminal 102 b (e.g. the cathode). The first terminal 102 a of the laserdiode 102 may be coupled to a first reference voltage V1. In the exampleshown in FIG. 1, the first terminal 102 a of the laser diode 102 iscoupled to the first reference voltage V1 via a safety switch 108. Inthe example shown in FIG. 1, the safety switch 108 is implemented usinga p-type metal-oxide-semiconductor (PMOS) transistor, although otherimplementations of the safety switch 108 may be possible in otherembodiments.

The second terminal 102 b of the laser diode 102 may be coupled to afirst terminal 104 a of the driver circuit 104. A second terminal 104 bof the driver circuit 104 may be coupled to a second reference voltageV2. In some embodiments, the first reference voltage V1 may be apositive supply rail, while the second reference voltage V2 may be anegative supply rail or a ground voltage. As such, the first referencevoltage V1 may be greater than the second reference voltage V2.

The safety switch 108 may be configured to electrically decouple thelaser diode 102 from the first reference voltage V1 when a faultcondition occurs (e.g. when the second terminal 102 b of the laser diode102 is shorted to the first reference voltage V1 or the second referencevoltage V2). As an example, a safety control signal EN1 is provided tothe safety switch 108 such that the safety switch 108 is closed duringnormal operation, thereby coupling the laser diode 102 to the firstreference voltage V1. However, when a fault condition occurs, the safetycontrol signal EN1 may change amplitude, causing the safety switch 108to open, thereby decoupling the laser diode 102 from the first referencevoltage V1. In the embodiment shown in FIG. 1, the safety control signalEN1 is provided to a gate of the PMOS transistor implementing the safetyswitch 108. Consequently, in the example of FIG. 1, the safety controlsignal EN1 is LOW during normal operation, but goes HIGH in response tothe occurrence of a fault condition.

As shown in FIG. 1, the circuit 100 includes the driver circuit 104,which is configured to turn the laser diode 102 on and off in responseto a first control signal EN2 provided to the driver circuit 104. Thedriver circuit 104 includes a current source 110 and a first switch 112.In the example shown in FIG. 1, the first terminal 104 a of the drivercircuit 104 corresponds to a first terminal of the current source 110,and thus, the first terminal of the current source 110 is coupled to thesecond terminal 102 b of the laser diode 102. Furthermore, as depictedin FIG. 1, the first switch 112 is coupled between a second terminal ofthe current source 110 and the second reference voltage V2. The currentsource 110 is controlled by a current source control signal ISRC, whilethe first switch 112 is controlled by the first control signal EN2. Inthe embodiment depicted in FIG. 1, the current source 110 and the firstswitch 112 are each implemented using an n-typemetal-oxide-semiconductor (NMOS) transistor, although otherimplementations of the current source 110 and the first switch 112 maybe possible in other embodiments. In the example shown in FIG. 1, inorder to control the operation of the current source 110 and the firstswitch 112, the current source control signal ISRC and the first controlsignal EN2 are respectively provided to the gates of the NMOStransistors implementing the current source 110 and the first switch112.

For the embodiment shown in FIG. 1, the current source control signalISRC is HIGH during normal operation, thereby turning on or enabling thecurrent source 110. The current source 110 supplies, to the laser diode102, a bias current (e.g. a DC current) that is at least equal to athreshold current of the laser diode 102, but less than a lasingthreshold of the laser diode 102.

FIG. 2 shows an example of the first control signal EN2 provided to thefirst switch 112 of the driver circuit 104, in accordance with anembodiment. In particular, FIG. 2 shows a variation of the amplitude ofthe first control signal EN2 with time. As depicted in FIG. 2, from timeto time t1, the first control signal EN2 is at a first amplitude (e.g.LOW). In response to the first control signal EN2 being at the firstamplitude (e.g. LOW), the first switch 112 is open, thereby decouplingthe current source 110 and the laser diode 102 from the second referencevoltage V2. As such, current is prevented from flowing through the laserdiode 102, regardless of whether the safety switch 108 is closed orwhether the current source 110 is enabled. Consequently, the laser diode102 is turned off and does not emit light between time t0 to time t1.

In response to the first control signal EN2 being at the secondamplitude, the first switch 112 is closed, thereby coupling the currentsource 110 to the second reference voltage V2. Assuming that the safetyswitch 108 is closed and the current source 110 is enabled, currentflows through the laser diode 102 in response to the first controlsignal EN2 being at the second amplitude (e.g. HIGH_1). As aconsequence, the laser diode 102 is turned on and emits light for atleast a portion of the time between time t1 and time t3. As describedabove, the laser diode 102 may be a semiconductor laser (e.g. anelectrically pumped semiconductor layer) in which the active lasermedium is formed by a p-n junction of a semiconductor diode. As aconsequence of the first control signal EN2 transitioning from the firstamplitude to the second amplitude (e.g. from time t1 to time t2) andremaining at the second amplitude (e.g. from time t2 to time t3), thep-n junction of the laser diode 102 is forward-biased since the firstreference voltage V1 is greater than the second reference voltage V2.This, in turn, drives either holes or electrons or both across the p-njunction of the laser diode 102 so as to “inject” them into activelayers of the laser diode 102. This creates an optical output current inthe laser diode 102.

Whether light is emitted from the laser diode 102 may depend, at leastin part, on whether the optical output current in the laser diode 102 isgreater than a lasing threshold, which may be a threshold above whichlight is emitted from the laser diode 102. In the event that the opticaloutput current in the laser diode 102 is below the lasing threshold,light may not be emitted from the laser diode 102. Conversely, in theevent that the optical output current in the laser diode 102 is greaterthan the lasing threshold, light may be emitted from the laser diode102.

Assuming that the optical output current in the laser diode 102 isgreater than the lasing threshold, the electrons recombine with theholes to produce light, and optical feedback provided by oppositesemiconductor layers of the laser diode 102 allows resonance of some ofthe emitted light to produce coherent “lasing” through a surface (e.g.the top surface or the bottom surface) of the laser diode 102. In someembodiments, the intensity of the light emitted from the laser diode 102may be proportional to the optical output current flowing through thelaser diode 102.

Between time t3 and time t3 the first control signal EN2 transitionsfrom the second amplitude (e.g. HIGH_1) to the first amplitude (e.g.LOW). In some embodiments, the fall time (e.g. the time duration betweentime t4 and time t3) may be similar to the rise time of first controlsignal EN2. In response to the first control signal EN2 being at thefirst amplitude (e.g. LOW), the first switch 112 is open once again,thereby decoupling the current source 110 from the second referencevoltage V2. As such, current is prevented from flowing through the laserdiode 102, regardless of whether the safety switch 108 is closed orwhether the current source 110 is enabled. The laser diode 102 iseventually turned off (e.g. does not emit light) when the current in thelaser diode 102 falls below the lasing threshold. The laser diode 102remains off until a subsequent transition of the first control signalEN2 from the first amplitude to the second amplitude. The subsequenttransition of the first control signal EN2 is indicated in FIG. 2 as thepulse labeled 202.

Each of the pulses of the first control signal EN2 resembles a top-hatpulse, and in some embodiments, a signaling frequency of the firstcontrol signal EN2 is upwards of 1 GHz. In other words, in someembodiments, a time difference T between rising edges of consecutivepulses of the first control signal EN2 can be less than about 1 ns. Insuch examples, the total time duration of each pulse of the firstcontrol signal EN2 (indicated in FIG. 2 as time duration TP) is lessthan 1 ns. As such, in order for the driver circuit 104 to drive thelaser diode 102 so that higher data rates are effectively and robustlysupported, it may be desirable for the optical output current of thelaser diode 102 (hereinafter referred to as “output current”) toresemble the shape of top-hat pulses of the first control signal EN2. Inorder for this to occur, the output current of the laser diode 102 mayneed to have a rise time and a fall time that is comparable orsubstantially equal to the rise time and the fall time of the pulses ofthe first control signal EN2.

However, when the laser diode 102 is turned on (e.g. between time t1 andtime t3), the p-n junction of the laser diode 102 is forward-biased, andcharged carriers are stored at the second terminal 102 b of the laserdiode 102. Consequently, when the first control signal EN2 transitionsfrom the second amplitude (e.g. HIGH_1) to the first amplitude (e.g.LOW) (e.g. between the time t3 and the time t4), a diffusion capacitanceof the p-n junction of the laser diode 102 may act as a parasiticcapacitance that opposes the transition from a forward-biased p-njunction to a reverse-biased p-n junction. This parasitic diffusioncapacitance of the p-n junction of the laser diode 102 can detrimentallyincrease the time needed for the current in the laser diode 102 to fallbelow the lasing threshold, and consequently, the time needed for thelaser diode 102 to turn off. In other words, the turn-off time of thelaser diode 102 may be substantially greater than the fall time of thefirst control signal EN2 due to the buildup of charged carriers at theanode of the laser diode 102.

As such, in order to ensure that the output current of the laser diode102 has a fall time that is comparable or substantially equal to therise time and the fall time of the pulses of the first control signalEN2, it may be desirable to remove the charged carriers stored at thesecond terminal 102 b of the laser diode 102 when the first controlsignal EN2 transitions from the second amplitude (e.g. HIGH_1) to thefirst amplitude (e.g. LOW). Removing the charged carriers stored at thesecond terminal 102 b of the laser diode 102 reduces the parasiticdiffusion capacitance of the p-n junction of the laser diode 102. Thismay, in turn, decrease the turn-off time of the laser diode 102, therebyincreasing the maximum data rate the laser diode 102 can support.

Furthermore, it may be desirable to still maintain a non-zero voltageacross the laser diode 102 when the laser diode 102 is in the “off”state (e.g. when it is not emitting light). The non-zero voltage acrossthe laser diode 102 when the laser diode 102 is in the “off” stateallows the laser diode 102 to be turned on at a faster rate when thefirst control signal EN2 transitions from the first amplitude (e.g. LOW)to the second amplitude (e.g. HIGH_1) (e.g. between time t1 and timet2). This may, in turn, decrease the turn-on time of the laser diode102, which can also increase the maximum data rate the laser diode 102can support.

By implementing both the removal of charged carriers stored at thesecond terminal 102 b of the laser diode 102 when the first controlsignal EN2 transitions from the second amplitude (e.g. HIGH_1) to thefirst amplitude (e.g. LOW) and the maintenance of a non-zero voltageacross the laser diode 102 when the laser diode 102 is in the “off”state, the rise time and the fall time of the output current of thelaser diode 102 can be comparable or substantially equal to the risetime and the fall time of the pulses of the first control signal EN2.This, in turn, can result in the output current of the laser diode 102resembling the top-hat shape of pulses of the first control signal EN2.

The snubber circuit 106 achieves the above-described removal of chargedcarriers stored at the second terminal 102 b of the laser diode 102 whenthe first control signal EN2 transitions from the second amplitude (e.g.HIGH_1) to the first amplitude (e.g. LOW). The snubber circuit 106 alsoachieves the above-described maintenance of a non-zero voltage acrossthe laser diode 102 when the laser diode 102 is in the “off” state. Insome embodiments, the snubber circuit 106 is included, on-chip, with thedriver circuit 104 and the laser diode 102.

As shown in FIG. 1, the snubber circuit 106 is coupled between the firstreference voltage V1 and the second terminal 102 b of the laser diode102. The snubber circuit 106 includes a second switch 114 having a firstterminal 114 a, a second terminal 114 b, and a third terminal 114 c. Thesnubber circuit 106 also includes a capacitive element 116 having afirst terminal 116 a and a second terminal 116 b. In some embodiments,the capacitive element 116 may include one or more capacitors arrangedin series or parallel, or both (e.g. such as in a capacitive network).In some embodiments, the capacitive element 116 may be trimmable and mayhave a plurality of selectable capacitance values. In such embodiments,the capacitive element 116 may assume one or more discrete capacitancevalues corresponding to the plurality of selectable capacitance values,for purpose of tuning a matching capacitance values, with respect to,lasing diode junction capacitance and/or the interaction of parasiticelement, such as bond wire inductance, in obtaining optimalperformance/tradeoff.

As depicted in the embodiment of FIG. 1, the first terminal 116 a of thecapacitive element 116 and the third terminal 114 c of the second switch114 are coupled to the first reference voltage V1. Furthermore, thesecond terminal 116 b of the capacitive element 116 is coupled to thesecond terminal 114 b of the second switch 114. Additionally, the firstterminal 114 a of the second switch 114 is coupled to the secondterminal 102 b of the laser diode 102 and the first terminal 104 a ofthe driver circuit 104.

The second switch 114 is configured to controllably couple thecapacitive element 116 to the second terminal 102 b of the laser diode102. In particular, the operation of the second switch 114 is controlledby a second control signal EN3. Removal of charged carriers stored atthe second terminal 102 b of the laser diode 102 when the first controlsignal EN2 transitions from the second amplitude (e.g. HIGH_1) to thefirst amplitude (e.g. LOW) and the maintenance of a non-zero voltageacross the laser diode 102 when the laser diode 102 is in the “off”state are achieved by dynamically controlling the snubber circuit 106using the second control signal EN3.

In the example shown in FIG. 1, the second switch 114 includes a PMOStransistor coupled between the second terminal 114 b and the thirdterminal 114 c of the second switch 114. The second switch 114 alsoincludes a CMOS transistor coupled between the second terminal 114 b andthe first terminal 114 a of the second switch 114. In particular, in theexample shown in FIG. 1, a drain of the PMOS transistor corresponds tothe third terminal 114 c of the second switch 114; the source of thePMOS transistor and the drain of the CMOS transistor are tied to formthe second terminal 114 b of the second switch 114; and the source ofthe CMOS transistor corresponds to the first terminal 114 a of thesecond switch 114.

FIG. 2 shows an example of the second control signal EN3 provided to thesecond switch 114 of the snubber circuit 106, in accordance with anembodiment. In particular, FIG. 2 shows a variation of the amplitude ofthe second control signal EN3 with time and relative to the timing ofthe first control signal EN2. Under the control of the second controlsignal EN3, the capacitive element 116 is coupled to or decoupled fromthe second terminal 102 b of the laser diode 102. As an example, thesecond control signal EN3 is provided to the gates of the PMOS and CMOStransistors of the second switch 114. In response to the second controlsignal EN3 being at a first amplitude (e.g. LOW), the CMOS transistor isturned off while the PMOS transistor is turned on. Consequently, thesecond terminal 116 b of the capacitive element 116 is decoupled fromthe second terminal 102 b of the laser diode 102 when the second controlsignal EN3 is at the first amplitude (e.g. LOW). On the other hand, inresponse to the second control signal EN3 being at a third amplitude(e.g. HIGH_2), the CMOS transistor is turned on while the PMOStransistor is turned off. Consequently, the second terminal 116 b of thecapacitive element 116 is coupled to the second terminal 102 b of thelaser diode 102 when the second control signal EN3 is at the thirdamplitude (e.g. HIGH_2).

As shown in FIG. 2, the falling edge of the first control signal EN2 istemporally aligned with (e.g. synchronized with) the rising edge of thesecond control signal EN3. As an example, at time t3, the first controlsignal EN2 transitions from the second amplitude (e.g. HIGH_1) to thefirst amplitude (e.g. LOW), while the second control signal EN3transitions from the first amplitude (e.g. LOW) to the third amplitude(e.g. HIGH_2). At time t4, the first control signal EN2 reaches thefirst amplitude (e.g. LOW), while the second control signal EN3 reachesthe third amplitude (e.g. HIGH_2).

A result of the relative timings of the first control signal EN2 and thesecond control signal EN3 is that charged carriers stored at the secondterminal 102 b of the laser diode 102 are removed when the first controlsignal EN2 transitions from the second amplitude (e.g. HIGH) to thefirst amplitude (e.g. LOW), consequently decreasing the turn-off time ofthe laser diode 102. As an example, up until time t3, the second controlsignal EN3 is at the first amplitude (e.g. LOW). As such, the secondterminal 116 b of the capacitive element 116 is decoupled from thesecond terminal 102 b of the laser diode 102, but coupled (e.g. via thePMOS transistor of the second switch 114) to the first reference voltageV1. Consequently, the capacitive element 116 is charged up (e.g.pre-charged to first reference voltage V1) between time t0 and time t3and while the laser diode 102 is turned on and emits light. However,since the capacitive element 116 is effectively decoupled from the laserdiode 102 during this period of time, the rise time of the outputcurrent of the laser diode 102 is not detrimentally affected (e.g.increased) by the capacitive element 116 of the snubber circuit 106.

However, when the first control signal EN2 transitions, at time t3, fromthe second amplitude (e.g. HIGH_1) to the first amplitude (e.g. LOW),the second control signal EN3 also transitions from the first amplitude(e.g. LOW) to the third amplitude (e.g. HIGH_2). As a result, the CMOStransistor is turned on while the PMOS transistor is turned off.Consequently, the second terminal 116 b of the capacitive element 116 isdecoupled from the first reference voltage V1, but coupled (e.g. via theNMOS transistor of the second switch 114) to the second terminal 102 bof the laser diode 102. This, in turn, results in a discharge of thecapacitive element 116 and a rapid increase in the voltage of the secondterminal 102 b of the laser diode 102. In effect, this rapid increase inthe voltage of the second terminal 102 b of the laser diode 102 drawsout remaining charged carriers from the diffusion capacitance within theforward-biased p-n junction of the laser diode 102, thus leading to arapid turn off time for the laser diode 102 and improved turn offcharacteristics of the laser diode 102. In effect, the capacitiveelement 116 is decoupled from the laser diode 102 for a fast rising edgeand settling when laser diode 102 is turned on, while the capacitiveelement 116 is coupled to the laser diode 102 for charge removal and fora fast falling edge when laser diode 102 is turning off.

Additionally, when the second control signal EN3 transitions from thefirst amplitude (e.g. LOW) to the third amplitude (e.g. HIGH_2), thesource-to-drain resistance RDS of the NMOS transistor is arranged inseries with the capacitive element 116. This arrangement has a low-passfiltering effect, which serves to damp RLC parasitic ringing of theoutput current (e.g. lasing current) of the laser diode 102 at turn off.It is noted that the inductance of the RLC parasitic ringing may beparasitic inductances of bond wires.

FIG. 3 shows a comparison of the output currents of the laser diode 102with and without the snubber circuit 106, in accordance with anembodiment. The vertical axis of the plot shown in FIG. 3 indicates theamplitude in milliamperes (mA), while the horizontal axis indicates timein nanoseconds (ns). In example of FIG. 3, the output current 300denotes the output current of the laser diode 102 when snubber circuit106 is operated in accordance with the timing diagram shown in FIG. 2.In particular, output current 300 shows the output current of the laserdiode 102 when the capacitive element 116 is decoupled from the laserdiode 102 when laser diode 102 is turned on and when the capacitiveelement 116 is coupled to the laser diode 102 when laser diode 102 isturning off. The output current 302, on the other hand, denotes theoutput current of the laser diode 102 without snubber circuit 106. Inthe example shown in FIG. 3, it is assumed that the first referencevoltage V1 is between about 3 V and about 4 V (e.g. about 3.6 V), ajunction capacitance of the p-n junction of the laser diode 102 isbetween about 2 pF and about 3 pF (e.g. about 2.4 pF), and the biascurrent provided by the current source 110 is about 2 mA.

As shown in the plots of FIG. 3, the output currents 300 and 302 havesubstantially equal rise times. As an example, the output currents 300and 302 rise at a rate of about 2.4×10⁹ A/s. On the other hand, theoutput current 300 obtained when snubber circuit 106 is operated inaccordance with the timing diagram shown in FIG. 2 shows a steeperfalling edge compared to the output current 302 obtained when snubbercircuit 106 is not used. In other words, the output current 300 shows afaster turn off time (e.g. a steeper falling edge), compared to theoutput current 302 when snubber circuit 106 is operated in accordancewith the timing diagram shown in FIG. 2. Furthermore, parasitic ringingfollowing an undershoot 300-U in the fast falloff of the output current300 is effectively damped by the above-described low-pass filteringeffect that is achieved by having the source-to-drain resistance RDS ofthe NMOS transistor of the second switch 114 being arranged in serieswith the capacitive element 116. Even further, even though there may bean overshoot 300-O in the output current 300, such an overshoot 300-Omay also be eliminated by tuning the duration for which the secondcontrol signal EN3 is at the third amplitude HIGH_2.

As described above, the capacitive element 116 may be trimmable. Assuch, the capacitance value and the time duration for which the snubbercircuit 106 couples the capacitive element 116 to the second terminal102 b of the laser diode 102 may be selectable. The capacitance valueand the time duration may be selected for tuning an optimal response forboth damping the response and matching the junction capacitance of thelaser diode 102.

Another effect of using the capacitive element 116 of the snubbercircuit 106 as described above is that the laser diode 102 is notcompletely clamped off. In other words, a voltage difference between thefirst terminal 102 a and the second terminal 102 b of the laser diode102 is greater than zero, even when laser diode 102 is not emittinglight. Consequently, use of the snubber circuit 106 reduces the need topre-charge any parasitic node capacitance which exists within the lasingcurrent network. Thus, in effect, this contributes to a faster turn-onresponse for the laser diode 102.

Additionally, use of the capacitive element 116 of the snubber circuit106 as described above allows for charge recovery, where the dampedcharge removed from the p-n junction of the laser diode 102 and storedwithin the capacitive element 116 is pumped back to the supply (e.g. thefirst reference voltage V1) when the first control signal EN2 is toggledback to the second amplitude (e.g. HIGH_1).

As such, the above-described implementation and use of the snubbercircuit 106 in conjunction with the laser diode 102 may generate pulsesfrom the laser diode 102 that resemble top-hat pulses with symmetricalrise and fall times. At a system level, pulses from the laser diode 102that resemble top-hat pulses can aid in resolving an informationcarrying signal of a time-of-flight (ToF) return signal detected at areceiver (e.g. an SPAD-based optical receiver). In addition, theabove-described use of the snubber circuit 106 in conjunction with thelaser diode 102 produces an output current having a narrower pulse width(e.g. as shown in the comparisons of output currents 300 and 302 in FIG.3). The narrower pulse widths of the output current of the laser diode102 can lead to finer resolution of distances of a ToF signal. Thenarrower pulse widths of the output current of the laser diode 102 canincrease signaling bandwidth and can also alleviate the problem of tightwindowing in optical communications (e.g. when a narrow time window isneeded to determine ambient noise).

FIG. 2 shows that the amplitude of the first control signal EN2 ischanged between the first amplitude (e.g. LOW or 0 V) to the secondamplitude (e.g. indicated as HIGH_1). FIG. 2 also shows that theamplitude of the second control signal EN3 is changed between the firstamplitude (e.g. LOW or 0 V) to the third amplitude (e.g. indicated asHIGH_2). In some embodiments, the third amplitude HIGH_2 is greater thanthe second amplitude HIGH_1. As an example, the first switch 112 mayhave a power supply voltage between about 1 V and about 2 V. As such,the first control signal EN2 may reside in a first power domain wherethe second amplitude HIGH_1 is between about 1 V and about 2 V (e.g.about 1.1 V). On the other hand, the second switch 114 may have a powersupply voltage between about 3 V and about 4 V. As such, the secondcontrol signal EN3 may reside in a second power domain where the thirdamplitude HIGH_2 is between about 3 V and about 4 V (e.g. about 3.3 V).An example of a component that resides the second power domain an analogfront-end that generates signaling output.

In embodiments, where the third amplitude HIGH_2 is greater than thesecond amplitude HIGH_1, it may be desirable for a single circuit togenerate both the first control signal EN2 and the second control signalEN3 so that the synchronization between the first control signal EN2 andthe second control signal EN3 (as indicated in FIG. 2) may be achieved.For example, as described above, the first control signal EN2 may have asignaling frequency of at least about 1 GHz. Consequently, the secondcontrol signal EN3 may need to also have a signaling frequency of atleast about 1 GHz so that the falling edge of the first control signalEN2 is synchronized or temporally aligned to the rising edge of thesecond control signal EN3. A mixed-voltage circuit (e.g. a levelshifter) may be used to generate both the first control signal EN2 andthe second control signal EN3 so that the synchronization between thefirst control signal EN2 and the second control signal EN3 (as indicatedin FIG. 2) may be achieved.

FIG. 4 shows an example of a level shifter 400 that may be used togenerate the first control signal EN2 and the second control signal EN3,in accordance with an embodiment. In particular, the level shifter 400may be a charge pump level shifter. Instead of positive feedback, thelevel shifter 400 implements level shifting using charge couplingcapacitors (indicated in FIG. 4 as capacitors 2V5). The charge couplingcapacitors 2V5 are tolerant to higher operating voltage than the voltagein the first power domain (where the supply voltage may be about 1.1 V,as described above). The charge coupling capacitors 2V5 are utilized forcoupling a voltage level from the first power domain (where the supplyvoltage may be about 1.1 V) to the second power domain (where the supplyvoltage may be about 3.6 V).

As indicated in FIG. 4, devices that reside in the first power domain(labeled in FIG. 4 as GO1 devices) and devices that reside in the secondpower domain (labeled in FIG. 4 as GO2 devices) are protected withcascode devices that reside in the second power domain. The cascodedevices residing in the second power domain allow for level-shiftedvoltage signaling for the second power domain (e.g. the second controlsignal EN3) from signaling for the first power domain (e.g. the firstcontrol signal EN2). It is noted that the use of labels GO2 and GO1 arein line with nomenclature describing a fabrication process. For example,devices labeled GO2 have thick gate oxides and are tolerant to largeroperating voltages (e.g. to operating voltages that reside in the secondpower domain where the supply voltage may be about 3.6 V). Furthermore,devices labeled GO1 have thinner gate oxides (in comparison to GO2devices) and are tolerant to smaller operating voltages (e.g. tooperating voltages that reside in the first power domain where thesupply voltage may be about 1.1 V). In some cases, GO2 devices includeI/O devices that function as peripheral analog blocks interfacing a chipto the real world.

FIG. 5 shows a simulation plot of the first control signal EN2 and thesecond control signal EN3 generated using the level shifter 400 shown inFIG. 4, in accordance with an embodiment. The horizontal axis of theplots shown in FIG. 5 denotes time in microseconds, while the verticalaxis denotes voltage in Volts. As shown in FIG. 5, the first controlsignal EN2 transitions from 0 V to about 1.1 V, while the second controlsignal EN3 transitions from about 0 V to about 3.6 V. Furthermore, sincethe first control signal EN2 is being generated and used by levelshifter 400 to produce second control signal EN3, it is seen in FIG. 5that the shape of the second control signal EN3 resembles the top-hatshape of the first control signal EN2. Furthermore, the frequency of thesecond control signal EN3 is substantially equal to the frequency of thefirst control signal EN2. Consequently, the synchronization between thesecond control signal EN3 and the first control signal EN2, as shown inFIG. 2, may be achieved (e.g. by use of a simple delay line tosynchronize the falling edge of the first control signal EN2 with therising edge of the second control signal EN3). FIG. 5 also shows a curve502, which includes amplitude spikes that resemble dirac functions. Theamplitude spikes are capped at the supply voltage of the first powerdomain (e.g. about 1.1 V) and occur at times coinciding with the fallingand rising edges of the first control signal EN2.

FIG. 6 shows a flow diagram illustrating a method 600 of operating thelaser diode 102, in accordance with an embodiment. The method 600includes step 602, which includes forward-biasing the laser diode 102 inresponse to shifting an amplitude of the first control signal EN2provided to the first switch 112 from the first amplitude (e.g. LOW) tothe second amplitude (e.g. HIGH_1). The method 600 further includes step604, which includes decoupling a terminal 116 b of the capacitiveelement 116 from the cathode 102 b of the laser diode 102 in response toholding the amplitude of the second control signal EN3 provided to thesecond switch 114 at the first amplitude (e.g. LOW) when shifting theamplitude of the first control signal EN2 to the second amplitude (e.g.HIGH_1). The method 600 also includes step 606, which includes shiftingthe amplitude of the first control signal EN2 from the second amplitude(e.g. HIGH_1) to the first amplitude (e.g. LOW) to prevent current fromflowing through the forward-biased laser diode 102. The method 600additionally includes step 608, which includes shifting the amplitude ofthe second control signal EN3 from the first amplitude (e.g. LOW) to thethird amplitude (e.g. HIGH_2) to couple the terminal 116 b of thecapacitive element 116 to the cathode 102 b of the laser diode 102.

In summary, snubber circuit 106 draws out remaining charged carrierswithin a p-n junction of the laser diode 102, leading to a rapid turnoff time for the laser diode 102. The snubber circuit 106 also serves todamp RLC parasitic ringing of an optical output current generated by thelaser diode 102 at turn off. The snubber circuit 106 further allows fora non-zero voltage drop across the laser diode 102 even when laser diode102 is not emitting light, thereby reducing the need to pre-charge anyparasitic node capacitance that exists within the laser diode 102, thusleading to a faster turn-on response for the laser diode 102. Thesnubber circuit 106 additionally allows for charge recovery, where thedamped charge removed from the p-n junction of the laser diode 102 ispumped back to the power supply V1.

In an embodiment, a circuit includes a diode having a first terminalcoupled to a first reference voltage; a first controllable switchcoupled between a second terminal of the diode and a second referencevoltage; and a capacitive element having a first terminal coupled to thefirst reference voltage and a second terminal controllably coupled tothe second terminal of the diode.

In an embodiment, a circuit includes a current source; a laser diodecoupled between a first terminal of the current source and a firstreference voltage; and a first switch controlled by a first controlsignal and coupled between a second terminal of the current source and asecond reference voltage, the second reference voltage being less thanthe first reference voltage. The circuit further includes a capacitiveelement having a first terminal coupled to the first reference voltage;and a second switch having a first terminal coupled to a cathode of thelaser diode and a second terminal coupled to a second terminal of thecapacitive element. The second switch may be configured to couple thesecond terminal of the capacitive element to the first reference voltagein response to a second control signal being at a first voltage, and tocouple the second terminal of the capacitive element to the cathode ofthe laser diode in response to the second control signal being at asecond voltage different from the first voltage.

In an embodiment, a method includes forward-biasing a laser diode inresponse to shifting an amplitude of a first control signal provided toa first switch from a first amplitude to a second amplitude anddecoupling a terminal of a capacitive element from a cathode of thelaser diode in response to holding an amplitude of a second controlsignal provided to a second switch at the first amplitude when shiftingthe amplitude of the first control signal to the second amplitude. Themethod further includes shifting the amplitude of the first controlsignal from the second amplitude to the first amplitude to preventcurrent from flowing through the forward-biased laser diode; andshifting the amplitude of the second control signal from the firstamplitude to a third amplitude to couple the terminal of the capacitiveelement to the cathode of the laser diode.

Those of skill in the art will further appreciate that the variousillustrative logical blocks, modules, circuits, and algorithms describedin connection with the embodiments disclosed herein may be implementedas electronic hardware, instructions stored in memory or in anothercomputer-readable medium and executed by a processor or other processingdevice, or combinations of both. The devices and processing systemsdescribed herein may be employed in any circuit, hardware component,integrated circuit (IC), or IC chip, as examples. Memory disclosedherein may be any type and size of memory and may be configured to storeany type of information desired. To clearly illustrate thisinterchangeability, various illustrative components, blocks, modules,circuits, and steps have been described above generally in terms oftheir functionality. How such functionality is implemented depends uponthe particular application, design choices, and/or design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application,but such implementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a processor, a digital signal processor (DSP), anApplication Specific Integrated Circuit (ASIC), a field programmablegate array (FPGA) or other programmable logic device, discrete gate ortransistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. A processormay be a microprocessor, but in the alternative, the processor may beany conventional processor, controller, microcontroller, or statemachine. A processor may also be implemented as a combination ofcomputing devices, e.g., a combination of a DSP and a microprocessor, aplurality of microprocessors, one or more microprocessors in conjunctionwith a DSP core, or any other such configuration.

The embodiments disclosed herein may be embodied in hardware and ininstructions that are stored in hardware, and may reside, for example,in Random Access Memory (RAM), flash memory, Read Only Memory (ROM),Electrically Programmable ROM (EPROM), Electrically ErasableProgrammable ROM (EEPROM), registers, a hard disk, a removable disk, aCD-ROM, or any other form of computer-readable medium known in the art.An exemplary storage medium is coupled to the processor such that theprocessor can read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor. The processor and the storage medium may reside in anASIC.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A circuit, comprising: a diode having a firstterminal coupled to a first reference voltage; a first controllableswitch coupled between a second terminal of the diode and a secondreference voltage; a capacitive element having a first terminal coupledto the first reference voltage and a second terminal controllablycoupled to the second terminal of the diode; and a second controllableswitch having a first terminal, a second terminal, and a third terminal,wherein: the first terminal of the second controllable switch is coupledto the second terminal of the diode; the second terminal of the secondcontrollable switch is coupled to the second terminal of the capacitiveelement; and the third terminal of the second controllable switch iscoupled to the first reference voltage.
 2. The circuit of claim 1,wherein the diode comprises a laser diode.
 3. The circuit of claim 2,wherein the laser diode comprises a vertical cavity surface emittinglaser.
 4. The circuit of claim 1, further comprising a controllablecurrent source coupled between the second terminal of the diode and thefirst controllable switch.
 5. The circuit of claim 1, wherein the firstterminal and the second terminal of the diode comprise an anode and acathode of the diode, respectively.
 6. The circuit of claim 1, whereinan operation of the first controllable switch and the secondcontrollable switch are controlled by a first control signal and asecond control signal, respectively.
 7. The circuit of claim 6, whereina falling edge of the first control signal is synchronized with a risingedge of the second control signal.
 8. The circuit of claim 6, wherein atime difference between rising edges of consecutive pulses of the firstcontrol signal is less than or equal to about 1 ns.
 9. The circuit ofclaim 6, wherein a maximum amplitude of the first control signal is lessthan a maximum amplitude of the second control signal.
 10. The circuitof claim 9, wherein the maximum amplitude of the first control signal isbetween about 1 V and about 2 V, and wherein the maximum amplitude ofthe second control signal is between about 3 V and about 4 V.
 11. Acircuit, comprising: a current source; a laser diode coupled between afirst terminal of the current source and a first reference voltage; afirst switch controlled by a first control signal and coupled between asecond terminal of the current source and a second reference voltage,the second reference voltage being less than the first referencevoltage; a capacitive element having a first terminal coupled to thefirst reference voltage; and a second switch having a first terminalcoupled to a cathode of the laser diode and a second terminal coupled toa second terminal of the capacitive element, the second switch beingconfigured to couple the second terminal of the capacitive element tothe first reference voltage in response to a second control signal beingat a first voltage, and to couple the second terminal of the capacitiveelement to the cathode of the laser diode in response to the secondcontrol signal being at a second voltage different from the firstvoltage.
 12. The circuit of claim 11, wherein the first switch isconfigured to couple the current source to the second reference voltagewhen the second terminal of the capacitive element is coupled to thefirst reference voltage.
 13. The circuit of claim 11, wherein the firstswitch is configured to decouple the current source from the secondreference voltage when the second terminal of the capacitive element iscoupled to the cathode of the laser diode.
 14. The circuit of claim 11,wherein a rising edge of the second control signal is temporally alignedto a falling edge of the first control signal.
 15. The circuit of claim11, wherein the second switch comprises a first transistor having afirst conductivity type and a second transistor having a secondconductivity type, the first transistor having a drain coupled to thefirst reference voltage and a source coupled to the second terminal ofthe capacitive element, the second transistor having a drain coupled tothe second terminal of the capacitive element and a source coupled tothe cathode of the laser diode, and wherein the second control signal isprovided to gates of the first transistor and the second transistor. 16.The circuit of claim 15, wherein the first transistor and the secondtransistor comprise a PMOS transistor and an NMOS transistor,respectively.
 17. A method, comprising: forward-biasing a laser diode inresponse to shifting an amplitude of a first control signal provided toa first switch from a first amplitude to a second amplitude; decouplinga terminal of a capacitive element from a cathode of the laser diode inresponse to holding an amplitude of a second control signal provided toa second switch at the first amplitude when shifting the amplitude ofthe first control signal to the second amplitude; shifting the amplitudeof the first control signal from the second amplitude to the firstamplitude to prevent current from flowing through the forward-biasedlaser diode; and shifting the amplitude of the second control signalfrom the first amplitude to a third amplitude to couple the terminal ofthe capacitive element to the cathode of the laser diode.
 18. The methodof claim 17, wherein shifting the amplitude of the second control signalfrom the first amplitude to the third amplitude coincides in time withshifting the amplitude of the first control signal from the secondamplitude to the first amplitude.
 19. The method of claim 17, whereinthe third amplitude is greater than the second amplitude.
 20. The methodof claim 19, wherein the third amplitude is about 3.6 V and the secondamplitude is about 1.1 V.
 21. The method of claim 17, wherein the firstamplitude is about 0 V.
 22. The method of claim 17, wherein holding theamplitude of the second control signal provided to the second switch atthe first amplitude couples the terminal of the capacitive element to afirst reference voltage.